Molybdenum (Mo) metal is used for various specialty applications which require its unusual properties. The melting point of molybdenum is 2630.degree. C., over 1000.degree. C. higher than iron, which permits using molybdenum for furnace parts, rocket nozzles and other high-temperature applications wherein most metals would melt or fail. Molybdenum also possesses exceptional resistance to corrosion by mineral acids when exposed to such acids in non-oxidizing conditions.
The mechanical properties of an article fabricated from molybdenum typically depend upon the conditions which are used to shape or work the metal. For best results, molybdenum is worked at a temperature below its recrystallization temperature, thereby avoiding recrystallization and grain growth within the article. When recrystallization is allowed to occur, molybdenum has a tendency to become brittle at relatively low temperatures, e.g., near room temperature and below. Recrystallization becomes particularly difficult to avoid should the manufacturing process employ brazing or welding because temperatures, which are sufficient to induce recrystallization, exist locally at the brazing or welding site. The tendency of recrystallized molybdenum to become brittle is a deterrent to its use in many applications.
Another difficulty arises when commercial molybdenum extrusion products are used for applications which require that rods, tubes or pipes be joined to extend their length, i.e., to form continuous lengths, and that the joined products be bent into a coil or curved object. While the ductility of such commercial molybdenum extrudates is usually satisfactory in the longitudinal direction, the transverse ductility is typically unacceptable, if not zero, which causes failure as a result of stress cracking when the extrudate is bent.
Molybdenum has been alloyed with rhenium (Re), which is a metal with a 3180.degree. C. melting point. MoRe alloy pieces are conventionally welded by techniques which minimize welding temperatures, i.e., to avoid recrystallization, such as electron beam welding under an inert shielding gas or a high vacuum.